Historically as fuel was being dispensed into a vehicle's fuel tank, typically from an underground storage tank (UST), vapor in the vehicle's fuel tank would escape into the atmosphere. In order to prevent this, Stage II vapor recovery systems were developed to collect this vapor and return it to the UST.
Stage II vapor recovery systems recover fuel vapor released from a vehicle's fuel tank as fuel is being dispensed into the vehicle's fuel tank. As is known, Stage II vapor recovery systems may be a balance type system or a vacuum-assist type system. Stage II vapor recovery systems typically are only installed in urban areas where the escaping fuel vapors can pose a greater threat to the environment.
It is desirable to detect whether there is a leak in the vapor recovery system. However current procedures typically require one to first pressurize the system to a predetermined pressure.